It is well known to try to equalize power levels across all channels in a WDM (wavelength division multiplexed) optical transmission system or network. Any variation in power levels can result in bit errors from distortions caused by non linearities if the power is too high. If the power is too low, bit errors can result from noise at the receiver. Providing a margin in the optical power budget to allow for variations, adds to system costs. It is also known to provide end to end dynamic gain equalization using variable optical attenuators or dynamic filters. Controlling optical amplifiers to reduce gain tilt inherent in the amplifiers is known from U.S. Pat. No. 5,818,629.
One type of non linearity, SRS (Stimulated Raman Scattering) which causes cross talk at high power levels, can also cause significant spectral power gradient (SPG) (that is variation of power with wavelength). This means that in optical transmission systems utilising WDM, short wavelength channels interact with long wavelength channels via Stimulated Raman Scattering (SRS). The effect of this is to increase the apparent span loss for short wavelength channels, and decrease it for long wavelength channels. The amount of this gradient varies with the total optical power of all the wavelengths. It is recognized in U.S. Pat. No. 6,275,313 that spectral distortion (in other words, SPG) introduced by stimulated Raman scattering (SRS) in an optical fiber transmission system is always linear on a dB/nm scale and depends solely on the total input power and not on the input power distribution. By maintaining the total input power to the fiber constant, the resulting gradient may be compensated for or canceled by using a fixed optical filter.